We have good experiences with both FKPs and PPS caps.
We use PPS where we want a compact layout and small values (like 150p).
We find silver mica a touch bright for analog filters.
Where space is not a restriction, we want best properties and larger values, we use FKP, or MKP.
Best is still styroflex.

qusp, how to measure and design the source and receiver resistors? My old 10 MHz Hameg might help for LRCLK but for MCLK (e.g. 49.152 MHz) even a 100 MHz scope only shows 1st and 2nd harmonic? I guess not many will be able to test and design suitable values for their specific application

And what about the effect of having 0R resistors compared to just a trace?

If I want to have 50 Ohm impedance for the clock lines, TXline gives me 0.75mm trace width (1mm FR4, 35µm, 0.15mm gap) if I have a solid ground plane on bottom layer and surround the lines with GND on top layer. Would you agree?

Just as a small upgrade another version

well we kind of need more information than the above and its difficult, because the exact thickness and dielectric constant of the FR4 vary from batch to batch and the dielectric constant also varies depending on frequency. You need to use Duroid or some other higher quality Rogers PTFE or ceramic substrate to get something more stable and allow more arbitrary rules of thumb.

.75 doesnt seem thick enough to me. for example with a trace thickness of 35µm, with 150µm gap and 1mm board which leaves us 815µm substrate. if we just give an estimation of Er = 4, I get 1.6mm trace width for 49Ω (1.555mm for exactly 50Ω)

here is a good calculator (use microstrip for this calc), which PCB layout package are you using? Eagle? does it have the ability to handle these calculations for you?

the zero ohm resistor.. hmm yeah I nearly mentioned those, I would think they are problematic, but I cant suggest an alternative, this is where flip-flops/clock buffers with 2 inputs might be handy?

I assumed a grounded coplanar waveguide (see attached diagram). It is not available in the online calculator in your link but i found a tool here. Does not yet correspond to my board, however thought about adding a second groundplane on toplayer (with lots of vias). By the way: I am using Target! as design tool (just got used to it).

Having said that, I am not really sure if it makes sense to go too deep into those things. As you said, to make it really good you have to control many parameters which will be quite hard in a hobby project like this. I'll try to make some good estimations to come close to 50Ohm and that should be enough

I'll try to order pcb's this week. Any special recommendation? makePCB was very cheap, but communication and quality were not really top-notch...

Hi! I just became aware of this thread...
Very nice work! I Really like the clean schematics and your strive for a good PCB layout on a two layer board. If you intend to order multiple PCBs (even for beta testing) please count me in for one.
Thanks!